1. Field of the Invention
The invention relates to photoinducing patterns in an optical fiber or waveguide, e.g. for the purpose of making passive optical components for wavelength division multiplexed (WDM) networks, and also for dense WDM (DWDM) networks.
The invention relates more particularly to apodizing a Bragg grating made using a Lloyd mirror.
A Bragg grating presents a refractive index that varies in alternating manner along the grating. This variation can be defined by the way the mean index varies along the grating and by the envelope of the curve representing index variations (the “index modulation envelope”).
2. Description of Related Art
It is known that the spectral response of a Bragg grating is the Fourier transform of said envelope.
If the envelope is uniform, as shown in FIG. 1, the spectral response is a cardinal sine as shown in FIG. 2. Such a cardinal sine turns out to be unusable as a filter since it is poorly selective in wavelength because of its side lobes.
If the modulated envelope of the grating has a Gaussian distribution with a mean index that also has a Gaussian distribution, as shown in FIG. 3, then the spectral response is Gaussian as shown in FIG. 4. Nevertheless, the Gaussian mean index creates a Fabry-Perot interferometer which disturbs the spectral response.
The index distribution which is considered as being ideal is a modulation envelope of Gaussian type distribution with a mean index that is constant as shown in FIG. 5. Such an index distribution gives a Gaussian spectral response as shown in FIG. 6.
Photoinducing by means of a Lloyd mirror, i.e. interferometric photoinduction with wave front separation, is performed in known manner by means of a setup as shown in FIG. 7, the setup comprising a polarized laser source 100 which generates a beam 120 impinging obliquely on a Lloyd mirror 200 placed perpendicularly to and in the immediate vicinity of the fiber 300 in which photoinduction is to take place. The laser beam 120 presents a transverse intensity distribution that is substantially Gaussian, with its maximum lying on a central axis X of the beam.
The fiber 300 is mounted on a support 250 positioned relative to the Lloyd mirror 200 in such a manner that the central axis X of the laser beam 120 reaches the end of the mirror 200 that is adjacent to the fiber 300.
The beam 120 thus presents a bottom half 122 situated beneath its axis X in which light rays are reflected by the mirror 200, and a top half 124 situated above the axis X in which light rays are transmitted directly to the fiber 300 without being reflected.
The reflected portion 122 is superposed on the transmitted portion 124 so that the fiber 300 is illuminated by a folded beam, and thus by a diffracted light grating. The mean intensity in said grating tapers off going away from the central axis X of the laser beam 120.
This produces a mean intensity in the grating which varies in compliance with a half-Gaussian curve as shown in FIG. 8, with the maximum of the half-Gaussian lying on the central axis X of the beam 120, and with the attenuated edge of the half-Gaussian lying in the margins of the laser beam 120 that have been folded onto each other.
The index distribution that is obtained presents an average that varies along the fiber 300 in Gaussian manner and there is no Fabry-Perot interferometer in its spectral response, i.e. there are no oscillations since there is an index discontinuity in the center of the grating.
The modulated envelope of the index is likewise a half-Gaussian with its apex corresponding to the location along the fiber 300 where it comes flush with the mirror 200.
Nevertheless, the semi-Gaussian distribution of the mean index creates a pitch that varies from the center to the edge of the grating, which is known as “chirp”. This chirp is to be found in the spectral response where it has a value corresponding substantially to a pitch difference of πn of such a chirped index distribution.
The reflected frequency peak is broadened by the value of the chirp.
For applications which require the grating to reflect a very fine frequency peak (in particular for DWDM applications), this index distribution turns out to be insufficient.